I am amazed at how hard they are to find.
Most brackets on the market have two problems. First, they often assume a railing wider than 1.5". Secondly, they assume that the vertical supports for the railing will be 18" or so apart. They don't work on my deck. When I try them, the planter invariably swings precariously.
The brackets I currently use are awesome, but I can't find them any place.
Here are some images.
The great thing about this design is the tail at the bottom. While the tray support the weight of the planter, and the hook puts the weight on the railing, the tail at the bottom keeps the planter level. Because it leans against the rail below, it prevents the window box from swinging.
I've been to nurseries, hardware stores, general merchandise stores and still have had not luck finding them.
I got my first set when I bought some self-watering window boxes from Gardeners.com. They came with the window box. Unfortunately, they don't appear to sell them separately, and $40 for the window box seems a bit steep, especially since I have enough window boxes.
Any suggestions? Perhaps it's time to seek out someone who knows how to build things like this.